Process for preparing filamentary microtapes of labyrinthian cross section



P 3, 1968 L. E. LEF'EVRE ETAL 3,

PROCESS FOR PREPARING FILAMENTARY MICROTAPES OF LABYRINTHIAN CROSS SECTION Filed July 15, 1965 m mvsN' roRs.

L log 6. L efew-e Ro erfJ. Maf/w'eson F/qyd E. Romesbery HTT RNE Y6 United States Patent ABSTRACT OF THE DISCLOSURE This application discloses and claims a process for forming filamentary microtapes of labyrinthian cross section comprising the passage of an unoriented, flat ribbon of an organic, thermoplastic, resinous material through stationary restricting means of narrower width than said ribbon withdrawing the so formed microtape downwardly from the restricting means at an angle of at least 15 degrees followed by orienting the microtape.

This invention relates to a process for preparing filamentary microtapes of labyrinthian cross section from fiat tapes and ribbons. More particularly, it relates to such a process which provides exceptional control of the width, denier, and cross-sectional configuration of said microtapes.

This application is a continuation-in-part of US. Ser. No. 420,761, filed Dec. 23, 1964, which was a continuation-in-part of U.S. Ser. No. 198,401, filed May 18, 1962, which was a continuation-in-part of US. Ser. No. 30,250, filed May 19, 1960, all of said applications being now abandoned.

For many applications and uses, it is desirable to have filamentary articles of other than the traditional solid cylindrical configuration. For example, fiat tapes and ribbons would seem to be a suitable form for the manufacture of many woven and unwoven fabrics and similar articles. Such tapes and ribbons provide unusually high covering power per unit of volume and thus result in fabrics of less weight and of greater flexibility than those fabrics prepared from the prior filamentary articles of cylindrical section. However, when tapes or ribbons are employed in the usual weaving operations, it is usually necessary to twist, to plait, or otherwise to change the character of the fiat tape or ribbon prior to weaving. Those operations result in a filamentary article which is characterized by poor control of width, denier, and crosssectional geometry and result in fabrics having a hand that is little better than those prepared from the conventional cylindrical filaments.

One filamentary article that is finding increasingly wide usage in many applications is a filamentary microtape that is essentially rectangular, oval, or elliptical in peripheral silhouette. In cross section, however, these useful microtapes consist of a continuous transverse section of a tape or ribbon having the edges curled or having a transverse section of the tape or ribbon folded two, three, or more times back upon itself resulting, in effect, in a series of layers separated by air. This curling or folding results in a labyrinthian cross section. These microtapes of such labyrinthian section may be used directly in conventional textile operations and, when so used, are characterized in providing the high covering power per unit of volume inherent in such peripheral silhouette. In addition, these microtapes are relatively bulky due to the air spaces between the layers.

3,400,193 Patented Sept. 3, 1968 These filamentary microtapes of labyrinthian cross section, when prepared from the known organic, thermoplastic, resinous material-s, are of great desirability for several reasons in addition to those just mentioned. Tapes and ribbons from which the desired microtape may be formed are readily prepared. Thermoplastic microtapes permit the manufacture of fabrics which may be thermally formed or molded, heat sealed, embossed, calendered, and the like which greatly expands the utility of the fabrics. However, the thermoplastic materials frequently require orientation if the optimum properties are to be achieved in the filamentary article. In many cases, it is necessary or at least desirable to the very attainment of the tape or ribbon to orient the tape or ribbon prior to fabrication. However, the oriented form of a polymer is subject to certain difficulties, such as fibrillation in flat tape form, when that tape is subjected to great stress. It would be desirable to have a process for preparing filamentary microtapes of labyrinthian cross section from continuous, coherent tapes and ribbons.

Accordingly, it is the principal object of this invention to provide a process for preparing filamentary microtapes of labyrinthian cross section, which process provides substantially rigid and exact control of the microtape width and denier.

It is a further object to provide such a process utilizing a tape or ribbon of an organic, thermoplastic, resinous material.

It is a still further object of the invention to provide such a process which is utilizable with fused tapes or ribbons.

Another object is the provision of an apparatus for use in the process.

The above and related objects are achieved by means of the process comprising the sequential steps of passing an unoriented, continuous, coherent tape or ribbon of an organic, thermoplastic, resinous material through restricting means of narrower width than the width of said tape or ribbon while maintaining a minimum forwarding tension on said tape or ribbon, withdrawing the so-formed microtape from said restricting means at an angle of at least 15 degrees from the straight line projection of the tape through and past the restricting means and no greater than 15 degrees measured from a vertical plane through the path of said tape at its point of departure from said means, and then subjecting the soformed filamentary microtape to longitudinal stress to impart orientation therein.

The tapes useful in the present method maybe of any organic, thermoplastic, resinous material. As materials which may be advantageously used are the normally crystalline polymeric materials. These are the polymers which have a tendency to form crystallites or sites where small segments of a plurality of the polymer chains are oriented and held in position by secondary valence forces. This crystallite formation or crystallinity is usually visible when the polymers are examined by X-ray diffraction. Typical of the normally crystalline polymeric materials falling within the advantageous definition are the polymers and copolymers of at least percent by weight of vinylidene chloride with the remainder composed of one or more other rnonoethylenically unsaturated comonorners exemplary of which are vinyl chloride, vinyl acetate, vinyl propionate, acryl-onitrile, alkyl and aralkyl acrylates hav ing alkyl and aralkyl groups of up to about 8 carbon atoms, acrylic acid, acrylamide, vinyl alkyl ethers, vinyl alkyl ketones, acrolein, allyl esters and ethers, butadiene and chloroprene. Known ternary compositions also may be employed advantageously. Representative of such polymers are those composed of at least 70 percent by weight of vinylidene chloride with the remainder made up of, for example, acrolein and vinyl chloride, acrylic acid and acrylonitrile, alkyl acrylates and alkyl methacrylates, acrylonitrile and butadiene, acrylonitrile and itaconic acid; acrylonitrile and vinyl acetate, vinyl propionate, or vinyl chloride, allyl esters or ethers and vinyl chloride, butadiene and vinyl acetate, vinyl propionate, or vinyl chloride and vinyl ethers and vinyl chloride. Quaternary polymers of similar monomeric composition will also be known. It has been found that the normally crystalline copolymers composed of from about 92 to 99 percent by weight of vinylidene chloride and correspondingly from 8 to 1 percent by weight of acrylonitrile or of a lower alkyl acrylate have suitable polymerization characteristics, are well adapted for use in the manipulative steps in this process, and result in exceptionally useful filamentary articles. For these reasons these vinylidene chloride/acrylonitrile and vinylidene chloride/lower alkyl acrylate copolymers represent preferred species for use herein. It should be understood, however, that the process is not limited to the treatment of tapes and ribbons of normally crystalline polymers but that those formed from any non-elastic polymeric material may be used. There are many materials, such as polyvinyl chloride and polystyrene, which are capable of forming continuous, coherent articles which are orientable but do not normally form crystallites. The polymeric materials, whether crystalline or non-crystalline, may also include minor amounts of monomers, such as vinyl pyrrolidone, vinyl oxazolidinone, vinyl alkyl oxazolidinone, and the like, which are known to aid the dyereceptivity and other properties of fibrous materials. Likewise, it is possible for the polymers to contain interpolymerized light and heat stabilizers. Also operable in the present method are tapes and ribbons of polymeric materials, such as the polyolefins, including, for example, polyethylene, polypropylene, copolymers of ethylene and propylene, and polyisobutylene. Equally useful in the method are the tapes and ribbons formed from condensation polymers, such as the polyamides, including polyhexamethylene, diadipamide, and the polyesters, including polyethyleneterephthalate. Also of utility are the tapes and ribbons of rubber hydrochloride and thermoplastic synthetic cellulose derivatives, including cellulose esters, such as cellulose acetate and cellulose ethers, such as methyl celulose and hydroxypropyl methyl cellulose. It should be apparent that any organic, thermoplastic, resinous material which is capable of being formed into a fiat tape or ribbon will find utility in the present invention.

The useful tapes for the present method are flexible tapes usually of about 0.001 to 0.005 inch in thickness and of about 0.1 to 1 inch in width. The thickness and width to be used in any given instance will depend in large measure upon the end product desired. The above limits are those which would normally be associated with the manufacture of filamentary microtapes to be used in conventional textile fabrics. When it is desired to make filamentary microtapes of greater size than, for example, about 0.3 inch, it would usually be found desirable to employ other known fabrication means, such as thermal extrusion for their preparation. Wide sections of tape which are more accurately referred to as films are not handled conveniently in the present procedural steps. However, it should be understood that the process is not limited precisely to the 1-inch maximum, since useful articles may be prepared herefrom, although usually with less control of width and cross section than with the narrower tapes.

As mentioned, the method of the present invention involves the passage of a flat, fused tape in planar fashion through confining and restricting means to cause the formation of the desired labyrinthian cross section. By that is meant that these means are of smaller dimension than the width of the fiat tape passed therethrough. The confining and restricting means may take any given form wherein the cross-sectional area through which the tape passes is narrower than the flat tape itself. A particularly convenient and accordingly preferred means is a groove in a rigid stationary shaping device. With any given flat tape the shape and dimensions of the groove determine in large measure the shape and final dimensions of the microtape. The significance of this point will become more apparent as the description of the invention proceeds.

The advantages and benefits, as well as the operation of the herein claimed process, will be more apparent from the following description and the appended drawings which illustrate a preferred procedural sequence for carrying out they process .and an apparatus finding utility in said process.

In the'drawings: 1

FIGURE 1 represents an end elevation;'

FIGURE 2' represents a side elevation of a useful grooved "shoe;

FIGURES 3-5 represent typical groove sections;

FIGURE 6 represents a preferred embodiment of the grooved shoe;

FIGURE 7 represents a preferred groove configuration;

and i 1 FIGURE 8 represents schematically a typical procedural sequence for carrying out the process.

i In the embodiment illustrated in FIGURE 8, a flat, fused tape or ribbon -10 is fed through a suitable guide 11, around a first pair of snubbing rolls 12, and then through a groove located in a grooved shoe 13 or similar device. The tape or ribbon 10 is then passed around a second pair of snubbing rolls 14 which maintains a substantially constant minimum forwarding tension on the tape 10. By minimum forwarding tension is meant that force or tension required to draw the tape through the groove without causing significant molecular realignment in the tape. If the tape is permitted to pass through the groove in relaxed condition (i.e., less than minimum forwarding tension), the tape will be under practically no mechanical inducement to cause shaping. When greater tension than the stated minimum is used, a useful product will result but it will not be the anticapted product of this invention. The microtape will be subject to more variation in width, and the fold or curl will not set as well as when the present process is used. The so-formed microtape is then passed through still another pair of snu bing Polls 15 operated at a peripheral speed that is greater than that of the second snu-bbing rolls 14. Finally, the oriented mircotape 10 is wound on suitable collecting means 16.

In the illustrated embodiment, the tape 10 is caused to be oriented between the second 14 and third 15 snub- 'bing rolls. The peripheral speedsto be used will depend in large measure upon the polymer used in forming the tape. With the preferred normally crystalline vinylidene chloride polymers, this ratio must be at least 2 to 1 and preferably should be about 4 to 1. Stretch ratios to be used with other organic, thermoplastic, resinous materials vvill be known or may be determined by simple preliminary experiment. With the indicated normally crystalline vinylidene chloride polymers, orientation is accomplished from the supercooled state. With other polymtric materials and also when tapes and ribbons of relatively thick section or of rigid nature are employed, it is frequently desirable to heat the folded tapes during this stretching step. Some drawing down can also be accomplished by such a hot stretching step. Tape temperatures usually not exceeding about C. will permit the stretching inthese circumstances.

A preferred apparatus for shaping the microtapc is a stationary grooved shoe, such as that illustrated in FIG- URES 1 and 2 having grooves in a convex curvilinear surface. 1

The shape of the groove in the shoe determines to large extent the type of folding or curling which occurs and, combined with the dimensions of the groove relative to the tape, determines the amount of folding or curling that occurs. Typical grooves which provide the desired folding in the present invention are illustrated in FIG- URES 3, 4, 5 and 7 of the appended drawings. In general,

5 it has been found that grooves with divergent side walls, such as those of FIGURES 2-4, tend to encourage edge curling while grooves with parallel walls, such as that of FIGURE 5, tend to cause folding of the edge portions back upon the tape. Also as a general guide, the shape and dimensions of the bottom or root of the groove influences the resultant product. Thus, aflat-bottomed groove (FIGURE 3) having divergent side walls whereinthe bottom is a small fraction of the width of the flat tape used will encourage the production of a microtape having tightly rolled edges with the rolled edges touching. As the width of the groove root is increased, the resulting microtapes will tend to have the curled edges more proportionately separated until a point is reached where the microtape, when magnified, will have the cross-sectional appearance of a ribbon with curled beaded edges. As the divergence of the groove walls approaches parallel, there is a tendency for a combination of folding and curling to occur. This effect can also be attained with a groove of configuration, such as that of FIGURE 4. In this effect, there is a small amount of curling at the extremities of the tape and the tape then folded back upon itself. It should be apparent that the process and apparatus is subject to the preparation of a wide variety of cross-sectional configuration. Judicious selection of groove shape and dimensions, as well as of procedural conditions that will result in the desired microtape, can be made with but a few simple preliminary experiments.

It has been found that maximum mechanical effect is obtained when the tape enters the groove at somewhat of an are as opposed to a tangential entry. To achieve such an effect, a rod or bar may be aflixed to the heated shoe to bridge the groove transversely. When the tape is passed over the bar into the shoe, the preferred angle of entry will be obtained.

An embodiment which is preferred is the use of a tapered groove of diminishing cross-sectional area. This permits a progressive reduction in area of the restricting means and results in more controllable and orderly folding of the tape or ribbon. This embodiment is illustrated in FIGURE 7.

It has been found that the most reproducible results are achieved when the restricting means, such as the illustrated grooved shoe, is unheated. However, when the flat tape is formed from a material or is of a thickness that tends to resist the mechanical shaping forces, it has been found that heating the shaping device to a temperature below the fusion point will assist the shaping operation. At the fusion temperature, the tape may stick to the shaping device, causing breaks in the microtape.

The microtape must be drawn 01f the grooved shoe 13 at an acute angle (angle a in FIGURE 6) if the stated objectives of the invention are to be realized. It has been found that this deflection of the path of the tape is necessary if microtapes of uniform width are to be achieved. It has also been found that the angular deflection must be at least 15 degrees from the straight line projection (line X in FIGURE 6) of the path of the tape. When that angle is less than 15 degrees, the folding is irregular and non-uniform and the process is non-reproducible and erratic. Further, the angle should not exceed an angle (6 in FIGURE '6) of 15 degrees measured from a vertical plane through the path of said tape at its point of departure from the restricting means. If the deflection trespasses within that latter 15 degree area (angle in FIGURE 6), an undue stress is placed on the emerging microtape which may result in breakage and fibrillation. Preferably, the angle of deflection (a in FIGURE 6) should be from 15 to 30 degrees from the projected path to attain best results with the least amount of strain imposed on the microtape.

'Because of this acute angle withdrawal, it is virtually mandatory that the trailing edge of the groove in the shoe (represented as A) be rounded with a curvature of a very small radius of the order of about inch. This lessens the stress applied to the microtape by the preferred withdrawal.

It should be apparent that reproducibility of the folding is obtained only when the dimensions of the fused, flat tape are essentially constant and the shaping conditions, such as the level at which th tape enters the groove, the depth to which it travels within the groove, and other factors, are maintained substantially constant. These constant conditions are easily attained by use of the aforementioned 'bar afiixed to the shoe and further by drawing the tape through the grooved shoe under substantially constant minimum forwarding tension. This minimum forwarding tension is attained with the second set of snubbing rolls.

As mentioned, the process of the present invention permits the preparation of the microtapes from previously fused but unoriented tapes or ribbons. This process results in several benefits. It is capable of continuous operation and is capable of being fitted into an integrated scheme that would include the tape or ribbon making procedural sequence immediately preceding the present process. The microtapes resulting from the process of this invention are characterized by a higher tenacity and a better hand than the tape or ribbons from which they are formed. The microtapes are further characterized by uniform width and denier and by the absence of any frayed, uneven, or torn edges.

By means of the present method and apparatus it is possible to prepare microtapes of a wide variation in width/thickness ratio. Thus, microtapes having a width which is about two times greater than its thickness may be prepared with little deviation in this ratio. Such a filament will approximate the known monofilaments and fibers of oval cross section. However, with the same apparatus and with minor change in conditions, the width/ thickness ratio may be changed to 10 to 1 or greater. By merely changing the groove size, it is possible to attain width/ thickness ratios of 30 to l or higher. Microtapes of such width/thickness ratios closely approximate fine ribbons or tapes and yet be subject to unusually exact control of width and denier and have edges which are non-frayable and have high tear strength.

The present process also is readily adaptable to the production of tapes of different deniers with minimum change in apparatus and conditions. When two or more flat tapes or ribbons are stacked and passed through the herein claimed procedural sequence, there results a microtape of similar configuration to that of a single tape but with proportionately increased thickness and denier. The width of the so-laminated or plied tapes is relatively modestly increased over that of the single, flat tape.

It will be appreciated that width/thickness ratios, denier, and cross-sectional configuration may be influenced by tape width and thickness, as well as the abovementioned factors. The tape width can be adjusted with the slitting means or to a significant extent within limits by hot stretching the flat tape during or subsequent to fusion. Tape thickness can be varied during its preparation or by the aforementioned plying technique.

It can thus be seen that the process is susceptible in flexibility of operation to a diversity of microtape crosssectional configurations, deniers, and width/thickness ratios without the major retooling required by the prior known filament-making process when any change is contemplated.

The operation of the method of the process will be more apparent from the following illustrative examples wherein all parts and percentages are by weight.

EXAMPLE 1 A ribbon prepared by the continuous coagulation of a latex of a copolyrner of 97 percent vinylidene chloride and 3 percent acrylonitrile into a coagulum followed by fusion of the coagulum was made having a thickness of 0.002 inch and a width of 0.3125 inch. After fusion, the

ribbons were supercooled and guided into 0.04 inch stationary grooves having parallel sides. The ribbons prior to folding were 0.2 inch wide. The rate of travel through the fusion unit and over the supercooling roll was 21 feet per minute. Upon leaving the groove at an angle of about 15 degrees from the projected path of the ribbon, the folded ribbons were passed through a pair of snubber rolls operated at a surface speed of 29 feet per minute. The speed increase Was necessary to retain the ribbons in the grooves. Upon leavin the grooves, the folded ribbons were approximately 0.04 inch in width. After leaving the G roove Width Thick- Width/ Sample Width Denier (mils) ness thick- (inch) (mils) ness -1 Folded Tape 0.010 400 30 3. 3 9. 1 0-2 Folded Tape 0. 060 400 45 2. 2 21 0-3 Folded Tape 0 1093 1,200 80 3.6 22 0-4 Ribbons:

Three 0. 040 1, 650 33 6. 5. 1 Four 0. 040 2, 200 33 10. 0 3. 3 Five..- 0. 040 2, 750 33 13. 0 2. 5 Six 0. 040 3, 300 33 15. 5 2. 1, Unfolded Ta 400 100 0. 8 125 Do 1, 200 300 0. 8 375 The above results demonstrate the versatility and flexibility of the process and apparatus in preparing filamentary microtapes of labyrinthian cross section.

Using the indicated procedural sequence, thermally extruded ribbons of linear polyethylenes having melt indices between 0.3 and 1.3 were fabricated into folded microtapes of the dimensions and configurations noted above. In similar manner, filamentary microtapes were prepared from continuous, coherent ribbons of rubbed hydrochloride, polypropylene, and polyethylene terephthalate. In all instances, the process provided rigid control of the width, denier, and cross-sectional configuration within commercially acceptable specifications.

Ribbons of the above materials were passed through grooves having divergent side walls. The resulting microtapes had curled edges. The dimensional and denier control in all cases was satisfactory.

EXAMPLE 2 Linear polyethylene films of 0.001 inch in thickness and having melt indices of 6, 1.5 and 0.3 were slit into tapes /2 inch wide and subjected to the procedural sequence indicated in Example 1. The sequence was modified to include the passage of the folded tapes over a pair of heated rolls positioned between snubbing rolls 14 and 15. These rolls were heated to a temperature of about 130 C. The grooved shoe was heated to 80 C. Rolls 15 were heated to about 125 C. The grooves in the grooved shoe were 0.075 inch in width. In all instances, the process provided irregularly folded microtapes of uniform width, denier, and properties within commercially acceptable specifications.

EXAMPLE 3 A shaping device was constructed of steel in strict accordance with the teachings of US. 2,985,583, that is, the slot was of rectangular cross section with dimensions of 0.1 inch at the point of entry and 0.02 inch at the exit. The shaping device was capable of being heated and a means for measuring the temperature of the slot was provided. The shaping device was positioned between two pairs of snubbers whose speed of rotation could be varied. A supply of ribbons or strips was placed '8 outside of one snubber and a variable speed collecting means positioned outside the other snubber.

Polyethylene film strips of 1, 3 and 5 mils thickness and A inch wide (about 6 millimeters) were passed through the apparatus in the manner illustrated in FIGURE 8.

With the shaping device in the position that the base of the slot was in the same plane as the nips of the rollers on either side thereof are when situated about 1 inch above that plane and varying the temperature from 70 to 105 C. the following was found: using the 3 and 5 mil strips without any guiding force other than that shown, the ribbon rotated 90 degrees and traveled through the groove vertically with no folding whatever. By manually guiding the ribbon into the slot (that is, by holding the ribbon horizontal at the point of entry into the slot) to force some mechanical stress other than rotational, a non-uniform folding occurred at one edge of the ribbon or strip. The drawing speeds during these trials were varied from ratios of peripheral speeds of the drawing snubber to the entry snubber of 1:1 up to about 5:1. The products so produced were not useful textile filamentary products in the generally accepted sense of the word.

All of the trials with the 3 and 5 mil polyethylene film strips were repeated with the shaping device raised in various positions above the straight line between the snubbers. Only when that device was raised (about 3 /2 inches) so that the angle of withdrawal of the ribbon from the slot was at least 15 degrees from the projected path of the strips was any product produced that was of sufficient uniformity of dimension to be considered useful for textile purposes.

The 1 mil polyethylene ribbon was submitted to the same procedural treatment as the 3 and 5 mil ribbons.

When the 1 mil ribbon was manually guided through the shaping device positioned according to the Becker teachings, one edge of the ribbon folded over. This was accom-plished when the drawing stress applied on the ribbon was 2.-6:'1 and when the shaping device was heated to C.

When the 1 mil ribbon was processed through the same steps except that the shaping device was raised 3 /2 inches and using a temperature of C. and a draw ratio of 5: 1, a filamentary product of labyrinthian cross section having a width to thickness ratio of about 2.5:1 was prepared. The thickness of the filament was made up of about 7 to 8 layers of material with intervening voids,

Similar trials were attempted with 3 mil strips of polypropylene. The same results were noticed with this product as with the polyethylene.

- What is claimed is: I

1. A process for preparing microtapes of labyrinthian cross section characterized in having a width that is at least two times greater than its thickness, said process comprising the sequential steps of passing an unoriented, flat, continuous, coherent ribbon of an organic, thermoplastic, resinous material through open-sided, straightwalled restricting means, the gap between the straight walls being of narrower width than the width of said ribbon, said restricting means being so positioned that the shaping force is applied to both edges of only one face of said ribbon while maintaining minimum forwarding tension on said ribbon, withdrawing downwardly the so-formed microtape from said restricting means at an angle of at least 15 degrees from the straight line projection of the tape through and past the restricting means and no greater than 15 degrees measured from a vertical plane through the path of said tape at its point of departure from said means, and then subjecting the so-formed filamentary microtape to longitudinal stress to impart orientation therein.

2. The process claimed in claim 1 wherein said organic, thermoplastic, resinous material is a normally crystalline polymeric material.

3. The process claimed in claim 2 wherein said normally crystalline polymeric material is a vinylidene chloride polymer composed of at least 70 percent by weight vinylidene chloride with any remainder of at least one monoethylenically unsaturated comonomer.

4. The process claimed in claim 3 wherein said vinylidene chloride polymer is a copolymer of vinylidene chloride and acrylonitrile.

5. The process claimed in claim 3 wherein said vinylidene chloride polymer is a copolymer of vinylidene chloride and an alkyl acrylate having from 1 to 8 carbon atoms in the alkyl group.

6. The process claimed in claim 1 wherein said organic, thermoplastic, resinous material is a solid polyolefin.

7. The process claimed in claim 6 wherein said solid polyolefin is polyethylene.

8. The process claimed in claim 6 wherein said solid polyolefin is polypropylene.

9. The process claimed in claim 1 wherein said ribbon is about 0.001 to 0.005 inch in thickness and from 0.1 to 1 inch in width.

10. The process claimed in claim 1 wherein said restricting means is of progressively decreasing dimensions.

References Cited UNITED STATES PATENTS 5/1961 Becker 264--290 7/1965 Lefevre 264285 

